Methods and apparatus for controlling rotational speed of a rotor

ABSTRACT

A method for controlling a rotational speed of a rotor having at least one rotor blade, a rotor shaft, and an electrical generator coupled thereto. The method includes controlling a torque of the rotor shaft by controlling a torque of the electrical generator, alternating between changing an angle of pitch of the at least one rotor blade and maintaining the angle of pitch of the at least one rotor blade substantially constant, and maintaining a substantially constant rotational speed of the rotor during variable wind speeds above a predetermined rated wind speed.

BACKGROUND OF THE INVENTION

This invention relates generally to rotors, and more specifically tomethods and apparatus for controlling the rotational speed of a rotor.

During operation of at least some known wind turbines, rotor speed iscontrolled to prevent the wind turbine rotor from rotating at or above apredetermined speed that may damage components of the wind turbine. Forexample, at least some known wind turbines control rotor speed bypitching the rotor blades using a pitch drive system that changes anangle of the rotor blades, thereby changing the aerodynamic torque ofthe rotor. Since their introduction, wind turbines have continuouslyincreased in physical size and electrical power output. However, asrotor diameter and therefore rotor blade lengths increase, frictionwithin the pitch drive system may also increase, which may increase thetorque required by the pitch drive motor to pitch the rotor blades.Moreover, as rotor blade lengths increase it may also be more difficultto accurately control the rotor blade pitch because of increased gearbacklash within the pitch drive system. As such, the pitch drive systemmay be required to perform more pitching operations to accuratelycontrol the angle of the rotor blades, which may increase parasiticpower loses within the wind turbine and/or may increase an amount ofcooling required by the pitch drive system to prevent damage theretoand/or failure thereof. Moreover, pitching the rotor blades may increaseloads induced to a tower of the wind turbine due to the effect of bladepitch on rotor thrust.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a method is provided for controlling a rotational speedof a rotor having at least one rotor blade, a rotor shaft, and anelectrical generator coupled thereto. The method includes controlling atorque of the rotor shaft by controlling a torque of the electricalgenerator, alternating between changing an angle of pitch of the atleast one rotor blade and maintaining the angle of pitch of the at leastone rotor blade substantially constant, and maintaining a substantiallyconstant rotational speed of the rotor during variable wind speeds abovea predetermined rated wind speed.

In another aspect, a wind turbine includes a rotor having a hub, atleast one rotor blade coupled to the hub, and a rotor shaft coupled tothe hub for rotation therewith. The wind turbine also includes a bladepitch actuator coupled to the at least one rotor blade for controllingan angle of pitch of the at least one rotor blade, and an electricalgenerator coupled to the rotor shaft. The electrical generator isconfigured to couple to an electrical load. The wind turbine alsoincludes a frequency converter coupled to the electrical generator, anda processor coupled to the blade pitch actuator and coupled to thefrequency converter. The processor is configured to control a speed ofthe rotor by controlling a torque of the electrical generator using thefrequency converter to thereby control a torque of the rotor shaft, byalternating between changing an angle of pitch of the at least one rotorblade using the blade pitch actuator and maintaining the angle of pitchof the at least one rotor blade substantially constant, and bymaintaining a substantially constant rotational speed of said rotorduring variable wind speeds above a predetermined rated wind speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary wind turbine in accordancewith an embodiment of the present invention.

FIG. 2 is a partially cut-away perspective view of a portion of the windturbine shown in FIG. 1 in accordance with an embodiment of the presentinvention.

FIG. 3 is a schematic diagram of the wind turbine shown in FIGS. 1 and2.

FIG. 4 is a cross-sectional view of a portion of a rotor hub of the windturbine shown in FIGS. 1-3 illustrating an exemplary embodiment of apitch system for changing a pitch of a rotor blade of the wind turbineshown in FIGS. 1-3.

FIG. 5 is a flowchart illustrating an exemplary embodiment of a methodfor controlling a rotor speed of the wind turbine shown in FIGS. 1-3.

FIG. 6 is a graph illustrating an example of the method shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “blade” is intended to be representative of anydevice that provides reactive force when in motion relative to asurrounding fluid. As used herein, the term “wind turbine” is intendedto be representative of any device that generates rotational energy fromwind energy, and more specifically, converts kinetic energy of wind intomechanical energy. As used herein, the term “wind generator” is intendedto be representative of any wind turbine that generates electrical powerfrom rotational energy generated from wind energy, and morespecifically, converts mechanical energy converted from kinetic energyof wind to electrical power. As used herein, the term “windmill” isintended to be representative of any wind turbine that uses rotationalenergy generated from wind energy, and more specifically mechanicalenergy converted from kinetic energy of wind, for a predeterminedpurpose other than generating electrical power, such as, but not limitedto, pumping a fluid and/or grinding a substance.

FIG. 1 is a perspective view of an exemplary wind turbine 10 inaccordance with an embodiment of the present invention. FIG. 2 is apartially cut-away perspective view of a portion of wind turbine 10 inaccordance with an embodiment of the present invention. FIG. 3 is aschematic diagram of wind turbine 10. Wind turbine 10 described andillustrated herein is a wind generator for generating electrical powerfrom wind energy. However, in some embodiments, wind turbine 10 may be,in addition or alternative to a wind generator, any type of windturbine, such as, but not limited to, a windmill (not shown). Moreover,wind turbine 10 described and illustrated herein includes ahorizontal-axis configuration. However, in some embodiments, windturbine 10 may include, in addition or alternative to thehorizontal-axis configuration, a vertical-axis configuration (notshown). Wind turbine 10 may be coupled to an electrical load (notshown), such as, but not limited to, a power grid (not shown), forreceiving electrical power therefrom to drive operation of wind turbine10 and/or its associated components and/or for supplying electricalpower generated by wind turbine 10 thereto. Although only one windturbine 10 is shown in FIG. 1-3, in some embodiments a plurality of windturbines 10 may be grouped together, sometimes referred to as a “windfarm”.

Wind turbine 10 includes a body 16, sometimes referred to as a“nacelle”, and a rotor (generally designated by 18) coupled to body 16for rotation with respect to body 16 about an axis of rotation 20. Inthe exemplary embodiment, nacelle 16 is mounted on a tower 14. However,in some embodiments, in addition or alternative to tower-mounted nacelle16, wind turbine 10 includes a nacelle 16 adjacent the ground and/or asurface of water. The height of tower 14 may be any suitable heightenabling wind turbine 10 to function as described herein. Rotor 18includes a hub 22 and a plurality of blades 24 (sometimes referred to as“airfoils”) extending radially outwardly from hub 22 for converting windenergy into rotational energy. Although rotor 18 is described andillustrated herein as having three blades 24, rotor 18 may have anynumber of blades 24. Blades 24 may each have any length (whetherdescribed herein). For example, in some embodiments one or more rotorblades 24 are about 0.5 meters long, while in some embodiments one ormore rotor blades 24 are about 50 meters long. Other examples of blade24 lengths include 10 meters or less, about 20 meters, about 37 meters,and about 40 meters. Still other examples include rotor blades betweenabout 50 and about 100 meters long.

Despite how rotor blades 24 are illustrated in FIG. 1, rotor 18 may haveblades 24 of any shape, and may have blades 24 of any type and/or anyconfiguration, whether such shape, type, and/or configuration isdescribed and/or illustrated herein. One example of another type, shape,and/or configuration of rotor blades 24 is a ducted rotor (not shown)having a turbine (not shown) contained within a duct (not shown).Another example of another type, shape, and/or configuration of rotorblades 24 is a darrieus wind turbine, sometimes referred to as an“eggbeater” turbine. Yet another example of another type, shape, and/orconfiguration of rotor blades 24 is a savonious wind turbine. Evenanother example of another type, shape, and/or configuration of rotorblades 24 is a traditional windmill for pumping water, such as, but notlimited to, four-bladed rotors having wooden shutters and/or fabricsails. Moreover, wind turbine 10 may, in some embodiments, be a windturbine wherein rotor 18 generally faces upwind to harness wind energy,and/or may be a wind turbine wherein rotor 18 generally faces downwindto harness energy. Of course, in any embodiments, rotor 18 may not faceexactly upwind and/or downwind, but may face generally at any angle(which may be variable) with respect to a direction of the wind toharness energy therefrom.

Referring now to FIGS. 2 and 3, wind turbine 10 includes an electricalgenerator 26 coupled to rotor 18 for generating electrical power fromthe rotational energy generated by rotor 18. Generator 26 may be anysuitable type of electrical generator, such as, but not limited to, awound rotor induction generator. Generator 26 includes a stator (notshown) and a rotor (not shown). Rotor 18 includes a rotor shaft 30coupled to rotor hub 22 for rotation therewith. Generator 26 is coupledto rotor shaft 30 such that rotation of rotor shaft 30 drives rotationof the generator rotor, and therefore operation of generator 26. In theexemplary embodiment, the generator rotor has a rotor shaft 28 coupledthereto and coupled to rotor shaft 30 such that rotation of rotor shaft30 drives rotation of the generator rotor. In other embodiments, thegenerator rotor is directly coupled to rotor shaft 30, sometimesreferred to as a “direct-drive wind turbine”. In the exemplaryembodiment, generator rotor shaft 28 is coupled to rotor shaft 28through a gearbox 32, although in other embodiments generator rotorshaft 28 is coupled directly to rotor shaft 30. More specifically, inthe exemplary embodiment gearbox 32 has a low speed side 34 coupled torotor shaft 30 and a high speed side 36 coupled to generator rotor shaft28. The torque of rotor 18 drives the generator rotor to therebygenerate variable frequency AC electrical power from rotation of rotor18. Generator 26 has an air gap torque between the generator rotor andstator that opposes the torque of rotor 18. A frequency converter 38 iscoupled to generator 26 for converting the variable frequency AC to afixed frequency AC for delivery to an electrical load (not shown), suchas, but not limited to a power grid (not shown), coupled to generator26. Frequency converter 38 may be located anywhere within or remote towind turbine 10. For example, in the exemplary embodiment frequencyconverter 38 is located within a base (not shown) of tower 14.

In some embodiments, wind turbine 10 may include one or more controlsystems 40 coupled to some or all of the components of wind turbine 10for generally controlling operation of wind turbine 10 and/or as some orall of the components thereof (whether such components are describedand/or illustrated herein). For example, in the exemplary embodimentcontrol system(s) 40 are coupled to a rotor control 41 for generallycontrolling rotor 18. In the exemplary embodiment, control system(s) 40is mounted within nacelle 16. However, additionally or alternatively,one or more control systems 40 may be remote from nacelle 16 and/orother components of wind turbine 10. Control system(s) 40 may be usedfor, but is not limited to, overall system monitoring and controlincluding, for example, pitch and speed regulation, high-speed shaft andyaw brake application, yaw and pump motor application, and/or faultmonitoring. Alternative distributed or centralized control architecturesmay be used in some embodiments.

In some embodiments, wind turbine 10 may include a disc brake (notshown) for braking rotation of rotor 18 to, for example, slow rotationof rotor 18, brake rotor 18 against full wind torque, and/or reduce thegeneration of electrical power from electrical generator 26.Furthermore, in some embodiments, wind turbine 10 may include a yawsystem 42 for rotating nacelle 16 about an axis of rotation 44 forchanging a yaw of rotor 18, and more specifically for changing adirection faced by rotor 18 to, for example, adjust an angle between thedirection faced by rotor 18 and a direction of wind. Yaw system 42 maybe coupled to control system(s) 40 for control thereby. In someembodiments, wind turbine 10 may include anemometry 46 for measuringwind speed and/or wind direction. Anemometry 46, in some embodiments,may be coupled to control system(s) 40 for sending measurements tocontrol system(s) 40 for processing thereof. For example, and althoughanemometry 46 may be coupled to control system(s) 40 for sendingmeasurements thereto for controlling other operations of wind turbine10, anemometry 46 may send measurements to control system(s) 40 forcontrolling and/or changing a yaw of rotor 18 using yaw system 42.Alternatively, anemometry 46 may be coupled directly to yaw system 42for controlling and/or changing a yaw of rotor 18.

Wind turbine 10 may also include a plurality of sensors 48, each coupledto a corresponding blade 24 for measuring a pitch of each blade 24, ormore specifically an angle of each blade 24 with respect to a winddirection and/or with respect to rotor hub 22. Sensors 48 may be anysuitable sensor having any suitable location within or remote to windturbine 10, such as, but not limited to, optical encoders within pitchsystem 56 (described below). In some embodiments, sensors 48 are coupledto control system(s) 40 for sending pitch measurements to controlsystem(s) 40 for processing thereof. In the exemplary embodiment, windturbine 10 includes one or more sensors 50 coupled to generator rotorshaft 28 for measuring a speed of rotation of rotor shaft 28 and/or atorque of generator rotor shaft 28. Sensor(s) 50 may be any suitablesensor having any suitable location within or remote to wind turbine 10,such as, but not limited to, optical encoders, digital proximitysensors, strain gages, and/or tachometers. In some embodiments,sensor(s) 50 are coupled to control system(s) 40 for sending speedmeasurements to control system(s) 40 for processing thereof. Moreover,in the exemplary embodiment, wind turbine 10 includes one or moresensors 52 coupled to rotor shaft 30 for measuring a speed of rotationof rotor shaft 28 and/or a torque of rotor shaft 30. Sensor(s) 52 may beany suitable sensor having any suitable location within or remote towind turbine 10, such as, but not limited to, optical encoders, digitalproximity sensors, piezo-electric transducers, strain gages, and/ortachometers. In some embodiments, sensor(s) 52 are coupled to controlsystem(s) 40 for sending measurements to control system(s) 40 forprocessing thereof. Furthermore, in the exemplary embodiment, windturbine 10 includes one or more sensors 54 coupled to generator 26 formeasuring an electrical power output of generator 26. In someembodiments, sensor(s) 54 are coupled to control system(s) 40 forsending measurements to control system(s) 40 for processing thereof.Sensor(s) 54 may be any suitable sensor having any suitable locationwithin or remote to wind turbine 10, such as, but not limited to, Halleffect current transducers (CTs) and/or capacitive voltage transducers(CVTs).

Wind turbine 10 may also include one or more other sensors (not shown)coupled to one or more components of wind turbine 10 and/or theelectrical load, whether such component(s) are described or illustratedherein, for measuring parameters of such component(s). Such othersensor(s) may include, but are not limited to, sensors configured tomeasure displacements, yaw, pitch, moments, strain, stress, twist,damage, failure, rotor torque, rotor speed, an anomaly in the electricalload, and/or an anomaly of power supplied to any component of windturbine 10. Such other sensors may couple to any component of windturbine 10 and/or the electrical load at any location thereof formeasuring any parameter thereof, whether such component, location,and/or parameter is described and/or illustrated herein.

Wind turbine 10 includes a variable blade pitch system 56 forcontrolling, including but not limited to changing, a pitch angle ofrotor blades 24 (shown in FIGS. 1-3) with respect to a wind direction.FIG. 4 is a cross-sectional view of a portion hub 22 illustrating anexemplary embodiment of pitch system 56. Pitch system 56 may be coupledto control system(s) 40 (shown in FIGS. 1 and 2) for control thereby.Pitch system 56 includes one or more actuators (e.g., pitch drive gear58 and pitch ring gear 60, described below) coupled to hub 22 and blades24 for changing the pitch angle of blades 24 by rotating blades 24 withrespect to hub 22. The pitch actuators may include any suitablestructure, configuration, arrangement, means, and/or components, whetherdescribed and/or illustrated herein, such as, but not limited to,electrical motors, hydraulic cylinders, springs, and/or servomechansims.Moreover, the pitch actuators may be driven by any suitable means,whether described and/or illustrated herein, such as, but not limitedto, hydraulic fluid, electrical power, electro-chemical power, and/ormechanical power, such as, but not limited to, spring force. In theexemplary embodiment, the pitch actuators include a pitch drive gear 58that is coupled to a pitch ring gear 60, as shown in FIG. 4. Pitch ringgear 60 is coupled to blade 24 such that rotation of pitch drive gear 58rotates blade 24 about an axis of rotation 62 to thereby change thepitch of blade 24.

In some embodiments, the pitch actuators may be driven by energyextracted from rotational inertia of rotor 18 and/or a stored energysource (not shown) that supplies components of wind turbine 10, such as,but not limited to, control system(s) 40 and/or pitch system 56, energyduring an anomaly in the electrical load and/or power source coupled towind turbine 10. For example, an anomaly in the electrical load and/orpower source may include, but is not limited to, a power failure, anundervoltage condition, an overvoltage condition, and/or anout-of-frequency condition. As such, the stored energy source enablespitching of blades 24 during the anomaly. Although other stored energysources may be used, in some embodiments the stored energy sourceincludes hydraulic accumulators, electrical generators, stored springenergy, capacitors, and/or batteries. The stored energy sources may belocated anywhere within, on, adjacent to, and/or remote from windturbine 10. In some embodiments, the stored energy source stores energyextracted from rotational inertia of rotor 18, energy stored withinconverter 38, and/or other auxiliary energy sources such as, but notlimited to, an auxiliary wind turbine (not shown) coupled to windturbine 10, solar panels, and/or hydro-power installations.

Referring again to FIG. 3, in some embodiments, control system(s) 40include a bus 62 or other communications device to communicateinformation. One or more processor(s) 64 are coupled to bus 62 toprocess information, including information from anemometry 46, sensors48, 50, 52, and/or 54, and/or other sensor(s). Control system(s) 40 mayalso include one or more random access memories (RAM) 66 and/or otherstorage device(s) 68. RAM(s) 66 and storage device(s) 68 are coupled tobus 62 to store and transfer information and instructions to be executedby processor(s) 64. RAM(s) 66 (and/or also storage device(s) 68, ifincluded) can also be used to store temporary variables or otherintermediate information during execution of instructions byprocessor(s) 64. Control system(s) 40 may also include one or more readonly memories (ROM) 70 and/or other static storage devices coupled tobus 62 to store and provide static (i.e., non-changing) information andinstructions to processor(s) 64. Input/output device(s) 72 may includeany device known in the art to provide input data to control system(s)50 and/or to provide outputs, such as, but not limited to, yaw controland/or pitch control outputs. Instructions may be provided to memoryfrom a storage device, such as, but not limited to, a magnetic disk, aread-only memory (ROM) integrated circuit, CD-ROM, and/or DVD, via aremote connection that is either wired or wireless providing access toone or more electronically-accessible media, etc. In some embodiments,hard-wired circuitry can be used in place of or in combination withsoftware instructions. Thus, execution of sequences of instructions isnot limited to any specific combination of hardware circuitry andsoftware instructions, whether described and/or illustrated herein.Control system(s) 40 may also include a sensor interface 74 that allowscontrol system(s) 40 to communicate with anemometry 46, sensors 48, 50,52, and/or 54, and/or other sensor(s). Sensor interface 74 can be or caninclude, for example, one or more analog-to-digital converters thatconvert analog signals into digital signals that can be used byprocessor(s) 64.

FIG. 5 is a flowchart illustrating an exemplary embodiment of a method100 for controlling a speed of rotor 18 (shown in FIGS. 1-3). Method 100includes controlling 102 a torque of generator 26 (shown in FIGS. 2 and3) to thereby control a torque of rotor shaft 30 (shown in FIG. 2), andalternating between changing 104 an angle of pitch of one or more rotorblades 24 (shown in FIGS. 1 and 2) to facilitate controlling the speedof rotor 18 and maintaining 106 the angle of pitch of one or more rotorblades 24 substantially constant. Controlling 102 the generator torquemay include, but is not limited to, selecting generator torque, changingthe generator torque, and/or controlling a rate at which the generatortorque is changed. The value(s) of generator torque selected, changed,and/or controlled may be any suitable value(s) or range thereof, suchas, but not limited to +/−20% of a predetermined rated torque. Control102 of generator torque may be performed during any portion ofalternating between changing 104 and maintaining 106 pitch angle. Forexample, generator torque may be controlled 102 during a change 104 ofblade pitch angle and/or while pitch angle is maintained 106. In someembodiments, and for example, if controlling 102 generator torque issufficient to regulate a desired rotational speed of rotor 18, forexample a speed that generates a desired power output of generator 26,blade pitch angle may not be changed 102. Moreover, and for example, insome embodiments generator torque is controlled 102 simultaneously withchanging 104 pitch angle, for example, to facilitate preventingoperation of wind turbine 10 at speeds and/or torques above or belowpredetermined limits.

Changing 104 the pitch angle of blades 24 may include controlling a rateat which the pitch angle of one or more blades 24 is changed. Thevalue(s) of blade pitch angle changed 104 and/or maintained 106 may beany suitable value(s) or range thereof, such as, but not limited to,between about −5° and about +30°. In some embodiments, changing 104 ofblade pitch may include a dead band to accommodate, for example, gearbacklash. In some embodiments, the dead band is different depending upona direction of change of pitch angle.

Controlling 102 a torque of generator rotor shaft 28 and alternatingbetween changing 104 an angle of pitch of one or more rotor blades 24and maintaining 106 the angle of pitch of one or more rotor blades 24substantially constant may be performed using any suitable structure,process, and/or means. In the exemplary embodiment, method 100 controls102 generator torque and alternates between changing 104 an angle ofpitch of one or more rotor blades 24 and maintaining 106 the angle ofpitch of one or more rotor blades 24 substantially constant usingcontrol system(s) 40 (shown in FIGS. 2 and 3) and input from anemometry46 (shown in FIGS. 1 and 2), sensors 48 (shown in FIG. 1), 50 (shown inFIG. 2), 52 (shown in FIG. 2), and/or 54 (shown in FIG. 3), and/or othersensor(s). Generator torque may be controlled 102 using any suitablestructure, process, and/or means. In the exemplary embodiment, frequencyconverter 38 (shown in FIGS. 2 and 3) controls 102 the torque ofgenerator 26 by controlling 105 the air gap torque between the rotor andstator of generator 26 (shown in FIGS. 2 and 3). Moreover, in theexemplary embodiment, frequency converter 38 controls the torque ofgenerator rotor shaft 28 (shown in FIGS. 2 and 3) to thereby control thetorque of rotor shaft 30. However, in other embodiments, for examplewherein wind turbine 10 is a direct-drive wind turbine, frequencyconverter 38 may control the torque of a coupling between the generatorrotor and rotor shaft 30 to thereby control the torque of rotor shaft30. Blade pitch may be controlled 102 using suitable structure, process,and/or means. In the exemplary embodiment, blade pitch is controlled 102using variable blade pitch system 56.

Although method 100 may include controlling 102 generator torque andalternating between changing 104 and maintaining 106 blade pitch duringvariable speed operation of wind turbine 10 at or below a predeterminedrated wind speed (which may be based on a desired power output ofgenerator 26), in the exemplary embodiment method 100 controls 102generator torque and alternates between changing 104 and maintaining 106blade pitch to facilitate generally constant speed operation of windturbine 10 (maintaining a substantially constant speed of rotor 18) ator above the predetermined rated wind speed. In some embodiments, thewind speed is variable above the predetermined rated wind speed suchthat method 100 controls 102 generator torque and alternates betweenchanging 104 and maintaining 106 blade pitch to facilitate generallyconstant speed operation of wind turbine 10 during variable wind speedsover the predetermined rated wind speed.

FIG. 6 is a graph illustrating an example of method 100 (shown in FIG.5) controlling 102 generator torque and alternating between changing 104and maintaining 106 a blade pitch 107 at or above a predetermined ratedwind speed. More specifically, as wind speed increases above apredetermined rated wind speed of about 11 m/s, method 100 controls 102generator torque and alternates between changing 104 and maintaining 106blade pitch 107 to facilitate maintaining a generally constant rotorspeed 109 of about 1440 revolutions per minute (RPM). As such, above awind speed of about 11 m/s, a generally constant average power output108 of about 1550 kilowatts (KW) is generated from generator 26 (shownin FIGS. 2 and 3). Of course, FIG. 6 is only one example of method 100.Accordingly, wind speed, blade pitch 107, rotor speed 109, and poweroutput 108 are exemplary values only. Wind speed, blade pitch 107, rotorspeed 109, and power output 108 may have any other suitable value(s)depending upon, for example, a location of wind turbine 10, designparameters of wind turbine 10 overall and/or any component thereof,and/or other characteristics of wind turbine 10.

Referring now to FIGS. 5 and 6, in the exemplary embodiment alternatingbetween changing 104 and maintaining 106 blade pitch includes changing110 an angle of one or more rotor blades 24 in a plurality of successivesteps. For example, FIG. 6 illustrates an example of method 100 whereinblade pitch is increased in a plurality of successive steps 112 as windspeed increases above the predetermined rated wind speed of about 11m/s. Each successive step of blade pitch may be any suitable size, suchas, but not limited to 0.3°. Moreover, each successive step may increaseblade pitch or decrease blade pitch. In some embodiments, blade pitch ischanged in a plurality of successive equal steps that each increaseblade pitch. In some embodiments, blade pitch is changed in a pluralityof successive equal steps that each decrease blade pitch. In someembodiments, blade pitch is changed in a plurality of successive equalsteps, wherein some of the steps increase blade pitch and some of thesteps decrease blade pitch. In other embodiments, one or more steps isdifferent from one or more other steps. For example, as illustrated inFIG. 6, steps 112 include smaller steps between wind speeds of about 11m/s and about 13 m/s that transition to larger steps above about 13 m/sof wind speed.

To facilitate achieving a desired average power output of generator 26and to facilitate avoiding continuous operation of wind turbine 10 atspeeds and/or torques above or below predetermined limits, changing 104and/or maintaining 106 blade pitch angle may be based, at least in part,on a comparison 114 between a desired power output and an actual poweroutput of generator 26 generated during control 102 of generator torque.The actual power output of generator 26 may be measured or calculated.For example, in some embodiments control system(s) 40 receives a poweroutput of generator 26 from sensor 54 (shown in FIGS. 2 and 3) andchanges 104 or maintains 106 blade pitch angle based on the measuredpower output. In other embodiments, and for example, control system(s)40 calculates the power output of generator 26 based on a measuredtorque of generator 26 (in the exemplary embodiment a measured torque ofgenerator rotor shaft 28 received from sensor(s) 50, shown in FIGS. 2and 3) or a measured torque of rotor shaft 30 received from sensor(s) 52(shown in FIGS. 2 and 3), and based on a measured rotational speed ofrotor shaft 30 received from sensor(s) 52 or a measured rotational speedof generator 26 (in the exemplary embodiment a measured rotational speedof generator rotor shaft 28 received from sensor(s) 50).

In some embodiments, method 100 includes using a reduced pitch controlgain region on all pitch gains to reduce pitch activity when generatorspeed is adequately controlled by generator torque. More specifically, areduced pitch control gain region may be used to facilitate smoothing atransition between changing 104 and maintaining 106 blade pitch angle.For example, in some embodiments method 100 includes regulating at leastone transition between changing 104 and maintaining 106 blade pitchangle using a gain on a signal based on a difference between a measuredtorque of rotor 18 during control 102 and a substantially constantdesired mean reference torque. Furthermore, and for example, in someembodiments method 100 includes regulating at least one transitionbetween changing 104 and maintaining 106 blade pitch angle using a gainon a signal based on a difference between a measured speed of rotor 18during control 102 and a substantially constant desired mean referencespeed. Moreover, and for example, in some embodiments method 100includes regulating at least one transition between changing 104 andmaintaining 106 blade pitch angle using a gain on a signal based on adifference between a substantially constant desired mean reference powerand either a measured electrical power output of electrical generator 26or the product of a measured torque of rotor 18 during control 102 and ameasured rotational speed of rotor 18 during control 102. Regulating atleast one transition between changing 104 and maintaining 106 mayinclude determining whether to change blade pitch angle based on whetherthe difference between the constant desired mean reference and themeasured value (or product of a plurality of measured values) is above apredetermined threshold. For example, in some embodiments if thedifference between the constant desired mean reference and the measuredvalue (or product of a plurality of measured values) is above thepredetermined threshold, blade pitch angle may be changed 104 inproportion to such difference. Moreover, and for example, in someembodiments if the difference between the constant desired meanreference and the measured value (or product of a plurality of measuredvalues) is below the predetermined threshold, blade pitch angle may bemaintained 106.

The herein-described and/or illustrated embodiments are cost-effectiveand efficient for controlling rotor speed. More specifically, bycontrolling generator rotor shaft torque to control rotor speed andalternating between changing and maintaining blade pitch angle, theembodiments described and/or illustrated herein may facilitate reducingan amount of pitch movement required to control rotor speed. As such,the embodiments described and/or illustrated herein may facilitatereducing wear on pitch system components, may facilitate reducingparasitic power losses from the pitch system, and/or may facilitatereducing an interaction between blade pitch movements and rotor thrustinduced tower oscillations.

Although the embodiments described and/or illustrated herein aredescribed and/or illustrated with respect to a wind turbine, and morespecifically controlling rotor speed of a wind turbine, practice of theembodiments described and/or illustrated herein is not limited to windturbines. Rather, the embodiments described and/or illustrated hereinare applicable to controlling as speed of any rotor having one or moreblades.

Exemplary embodiments are described and/or illustrated herein in detail.The embodiments are not limited to the specific embodiments describedherein, but rather, components and steps of each embodiment may beutilized independently and separately from other components and stepsdescribed herein. Each component, and each step, can also be used incombination with other components and/or method steps.

When introducing elements/components/etc. described and/or illustratedherein, the articles “a”, “an”, “the”, “said”, and “at least one” areintended to mean that there are one or more of theelement(s)/component(s)/etc. The terms “comprising”, “including” and“having” are intended to be inclusive and mean that there may beadditional element(s)/component(s)/etc. other than the listedelement(s)/component(s)/etc.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method for controlling a rotational speed of a rotor having atleast one rotor blade, a rotor shaft, and an electrical generatorcoupled thereto, said method comprising: controlling a torque of therotor shaft by controlling a torque of the electrical generator;alternating between changing an angle of pitch of the at least one rotorblade in a plurality of successive steps and maintaining the angle ofpitch of the at least one rotor blade substantially constant; andmaintaining a substantially constant rotational speed of the rotorduring variable wind speeds above a predetermined rated wind speed.
 2. Amethod in accordance with claim 1 further comprising regulating at leastone transition between changing an angle of pitch of the at least onerotor blade and maintaining the angle of pitch of the at least one rotorblade substantially constant using a gain on a signal based on adifference between a measured torque of the rotor and a substantiallyconstant desired mean reference torque.
 3. A method in accordance withclaim 1 further comprising regulating at least one transition betweenchanging an angle of pitch of the at least one rotor blade andmaintaining the angle of pitch of the at least one rotor bladesubstantially constant using a gain on a signal based on a differencebetween a measured speed of the rotor and a substantially constantdesired mean reference speed.
 4. A method in accordance with claim 1further comprising regulating at least one transition between changingan angle of pitch of the at least one rotor blade and maintaining theangle of pitch of the at least one rotor blade substantially constantusing a gain on a signal based on a difference between a substantiallyconstant desired mean reference power and one of a measured electricalpower output of the electrical generator and the product of a measuredtorque of the rotor and a measured rotational speed of the rotor.
 5. Amethod in accordance with claim 1 wherein changing an angle of pitch ofthe at least one rotor blade further comprises changing an angle ofpitch of the at least one rotor blade based on a measured electricalpower output of the electrical generator.
 6. A method in accordance withclaim 1 wherein changing an angle of pitch of the at least one rotorblade further comprises changing an angle of pitch of the at least onerotor blade based on a measured torque of one of the electricalgenerator and the rotor shaft and a measured rotational speed of one ofthe electrical generator and the rotor shaft.
 7. A method in accordancewith claim 1 wherein changing an angle of pitch of the at least onerotor blade in a plurality of successive steps comprises changing theangle of pitch of the at least one rotor blade in a plurality ofsubstantially equal steps.
 8. A method in accordance with claim 1wherein changing an angle of pitch of the at least one rotor blade in aplurality of successive steps comprises changing the angle of pitch ofthe at least one rotor blade in a plurality of steps, wherein at leastone step of the plurality of steps is different from at least one otherstep of the plurality of steps.
 9. A method in accordance with claim 1wherein the rotor is a first rotor and controlling a torque of the rotorshaft by controlling a torque of the electrical generator furthercomprises controlling an air gap torque between a second rotor of theelectrical generator and a stator of the electrical generator.
 10. Amethod in accordance with claim 1 wherein controlling a torque of therotor shaft by controlling a torque of the electrical generator furthercomprises controlling a torque of the electrical generator duringgenerally constant speed operation of the rotor one of at and above apredetermined rated wind speed.
 11. A wind turbine comprising: a rotorcomprising a hub, at least one rotor blade coupled to said hub, and arotor shaft coupled to said hub for rotation therewith; a blade pitchactuator coupled to said at least one rotor blade for controlling anangle of pitch of said at least one rotor blade; an electrical generatorcoupled to said rotor shaft, said electrical generator configured tocouple to an electrical load; a frequency converter coupled to saidelectrical generator; and a processor coupled to said blade pitchactuator and coupled to said frequency converter, said processorconfigured to control a speed of said rotor by: controlling a torque ofsaid electrical generator using said frequency converter to therebycontrol a torque of said rotor shaft; alternating between changing anangle of pitch of said at least one rotor blade in a plurality ofsuccessive steps using said blade pitch actuator and maintaining theangle of pitch of said at least one rotor blade substantially constant;and maintaining a substantially constant rotational speed of said rotorduring variable wind speeds above a predetermined rated wind speed. 12.A wind turbine in accordance with claim 11 wherein said processor isconfigured to regulate at least one transition between changing an angleof pitch of said at least one rotor blade and maintaining the angle ofpitch of said at least one rotor blade substantially constant using again on a signal based on a difference between a measured torque of saidrotor and a substantially constant desired mean reference torque.
 13. Awind turbine in accordance with claim 11 wherein said processor isconfigured to regulate at least one transition between changing an angleof pitch of said at least one rotor blade and maintaining the angle ofpitch of said at least one rotor blade substantially constant using again on a signal based on a difference between a measured speed of saidrotor and a substantially constant desired mean reference speed.
 14. Awind turbine in accordance with claim 11 wherein said processor isfurther configured to regulate at least one transition between changingan angle of pitch of said at least one rotor blade and maintaining theangle of pitch of said at least one rotor blade substantially constantusing a gain on a signal based on a difference between a substantiallyconstant desired mean reference power and one of a measured electricalpower output of said electrical generator and the product of a measuredtorque of said rotor and a measured rotational speed of said rotor. 15.A wind turbine in accordance with claim 11 wherein said processor isconfigured to change an angle of pitch of said at least one rotor bladebased on a measured electrical power output of said electricalgenerator.
 16. A wind turbine in accordance with claim 11 wherein saidprocessor is configured to change an angle of pitch of said at least onerotor blade based on a measured torque of one of said electricalgenerator and said rotor shaft and a measured rotational speed of one ofsaid electrical generator and said rotor shaft.
 17. A wind turbine inaccordance with claim 11 wherein said processor is configured to changethe angle of pitch of said at least one rotor blade in a plurality ofsubstantially equal steps.
 18. A wind turbine in accordance with claim11 wherein at least one step of said plurality of steps is differentfrom at least one other step of said plurality of steps.
 19. A windturbine in accordance with claim 11 wherein said processor is configuredto control a torque of said electrical generator during generallyconstant speed operation of said wind turbine one of at and above apredetermined rated wind speed.